Flame-retardant cable

ABSTRACT

The present invention relates to a flame-retardant cable comprising a transmission element, a flammable element, and a flame-retardant coating layer surrounding said flammable element, and made of a material based on a polymer obtained from a polymerizable liquid composition containing at least a precursor for said polymer including functional groups selected from acrylates, methacrylates, epoxies, vinyl ethers, allyl ethers, and oxetanes,  
     wherein said material includes at least one phosphorous group.

RELATED APPLICATIONS

[0001] This application is related to and claims the benefit of priorityto French Patent Application No. 02 15065, filed on Nov. 29, 2002, theentirety of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] In known manner, the acquirers of electrical and/or opticalcables for transporting power and/or transmitting information, seek, inthe event of a fire, to avoid flame propagating along a cable, even iflaid vertically, and to prevent the insulating material that covers thecable core from dripping away when melted at high temperature.

[0003] European patent application EP 1 191 547 A1 describes a cablewhose insulating layer of polyethylene is coated in an outer layer thatis thin, preferably 5 micrometers (μm) to 50 μm thick, and serving tocombat fire propagation, for example. The coating is of polyacrylatepolymer formed by ultraviolet radiation.

[0004] Not all polyacrylate coatings are capable both of retardingdegradation of the cable and also of providing a cable which canconserve a degree of operability even when subjected directly to extremethermal stress such as flame or fire.

[0005] Furthermore, it is important for the outer coating also topossess good resistance to abrasion.

OBJECT AND SUMMARY OF THE INVENTION

[0006] The object of the invention is to devise a flame-retardant cable,which is preferably inexpensive, and which is quick and easy tomanufacture.

[0007] Another object of the invention is to devise such a cable thatwithstands abrasion.

[0008] To this end, the invention provides a flame-retardant cablecomprising a transmission element, a flammable element, and aflame-retardant coating layer surrounding said flammable element, andmade of a material based on a polymer obtained from a polymerizableliquid composition containing at least a precursor for said polymerincluding functional groups selected from acrylates, methacrylates,epoxies, vinyl ethers, allyl ethers, and oxetanes,

[0009] wherein said material includes at least one phosphorous group.

[0010] The phosphorous group(s) provide the flame-retardant propertiesof the coating of the invention.

[0011] In a preferred embodiment, the phosphorous group may bechemically bonded to said polymer, and in this embodiment, the precursorof said polymer may include at least one phosphorous group.

[0012] Advantageously, the material of the invention may be free fromhalogens, elements which are conventionally used as fire-retardingagents.

[0013] According to a characteristic, said flammable element may beselected from one or more of the following elements: an insulatinglayer; a sheathing layer; a reinforcing element; a tube for protectingoptical fibers; a grooved core; a serving string; a tape; and a braid.

[0014] When said flammable element is an insulating layer, saidinsulating layer may be made of a material selected from a halogen-freethermoplastic polymer, and preferably polyethylene which presents gooddielectric properties.

[0015] By way of example, in the field of telecommunications cableswhich are often laid vertically in ventilation ducts, polyethyleneprotected by the flame-retardant coating of the invention canadvantageously replace polytetrafluoroethylene (PTFE) or copolymers oftetrafluoroethylene and hexafluoropropylene (fluorinated ethylenepropylene copolymer FEP) that are better at withstanding fire, but thatpresent poorer dielectric properties.

[0016] The flammable element may include its own flame-retarding mineralfills, but that do not provide it with sufficient protection againstfire. In this configuration, the flame-retarding coating reinforces theability of said element to withstand fire.

[0017] In an embodiment of the invention, the transmission element isselected from an optical conductor and an electrical conductor.

[0018] In a first embodiment, said flame-retarding coating layer is madeby applying said polmerizable liquid composition on said flammableelement using a coating technique selected from spraying, dipping,impregnation, and application by means of a brush.

[0019] In a second embodiment, said flame-retarding coating layer isformed from a tape impregnated in said polymerizable liquid compositionand wound on said flammable element.

[0020] Advantageously, said polymerizable liquid composition may containa reactive diluant including an antiabrasion compound which ispreferably of bicyclic structure and contains at least one functionalgroup that is selectively reactive with one of the functional groups ofsaid polymer precursor.

[0021] In this way, the coating is not only highly flame-retardant, butalso withstands abrasion and presents good thermomechanical properties.In addition, the antiabrasion compound is easily miscible and makes thecomposition easier to apply.

[0022] Said composition may be polymerizable, for example, by actinicradiation (ultraviolet radiation, electrons, gamma rays, etc.).

[0023] The polymer precursor (monomer, oligomer) including acrylatefunctional groups and at least one phosphorous group is sold, forexample, by UCB Chemicals under the reference Ebecryl IRR 527.

[0024] The number of parts by weight of said antiabrasion compoundrelative to 100 parts by weight of said composition is preferably lessthan 95 and preferably lies in the range 10 to 30 in order to conservethe highly fire-retardant nature of the coating.

[0025] When said antiabrasion compound contains at least one acrylatefunctional group, the acrylate equivalent weight of said antiabrasioncompound is preferably greater than 80 and is preferably substantiallyequal to 210.

[0026] The term “acrylate equivalent weight” is used to mean the molarmass of the compound relative to the number of acrylate functions permolecule.

[0027] Thus, the coating layer presents good mechanical properties, inparticular good elasticity (high breaking elongation), and also improvedhardness.

[0028] Said liquid composition is preferably polymerizable by actinicradiation, and when said actinic radiation is of the ultraviolet type,said composition may include a photoinitiator, the number of parts byweight of said photoinitiator relative to 100 parts by weight of saidcomposition lying in the range 0.1 to 10, and preferably beingsubstantially equal to 3.

[0029] Said liquid composition is advantageously polymerizable by UVradiation and may contain:

[0030] 80 parts by weight of said polymer precursor, said precursorbeing a halogen-free oligomer;

[0031] 17 parts by weight of isobornyl acrylate; and

[0032] 3 parts by weight of a photoinitiator.

DESCRIPTION OF EXAMPLES

[0033] Other characteristics and advantages of the present inventionappear from the following description of examples given by way ofnon-limiting illustration.

[0034] Examples 1 and 2 relate to a liquid composition that ispolymerizable by radiation of the actinic type for the purpose of makinga flame-retardant coating layer of the invention for a power cable, adata cable, or a telecommunications cable.

EXAMPLE 1

[0035] Composition No. 1

[0036] 97 parts by weight of Ebecryl IRR 527 from UCB Chemicals, ahalogen-free polyester acrylate oligomer having two acrylate functionalgroups and phosphorous groups; and

[0037] 3 parts by weight of the photoinitiator DAROCUR1173 (commercialname) from CIBA.

EXAMPLE 2

[0038] Composition No. 2

[0039] 80 parts by weight of Ebecryl IRR 527;

[0040] 17 parts by weight of an isobornyl acrylate of bicyclic structuresuch as Genomer 1121 from RAHN, having an acrylate equivalent weightequal to 208; and

[0041] 3 parts by weight of DAROCUR1173 (commercial name)photoinitiator.

[0042] Table 1 gives the properties of a coating No. 1 made of amaterial based on a polymer obtained by using ultraviolet radiation topolymerize composition No. 1, and of a coating No. 2 made of a materialbased on a polymer obtained by using ultraviolet radiation to polymerizecomposition No. 2.

[0043] In these examples, the precursor also contains phosphorous groupsand the resulting polymer is chemically bonded to phosphorous groups.TABLE 1 Coating No. 1 Coating No. 2 Breaking stress 13.4 21.4 (MPa) at25° C. Breaking 46 43.1 elongation (%) Hardness (Buchholz <59 123method)

[0044] On heating to 60° C., composition No. 1 presents viscosity thatis equal to about 17,000 millipascal-seconds (mPa.s). Composition No. 2,which has viscosity of about 1206 mpa.s at 50° C., is easier to applythan composition No. 1 and leads to a coating No. 2 presenting goodelasticity and better resistance to abrasion.

[0045] The use of an isobornyl acrylate serves to improve the mechanicalproperties and the abrasion resistance of the coating. In addition, thiscompound with an acrylate functional group that is reactive with one ofthe acrylate functional groups of the oligomer makes it possible toachieve complete polymerization using a medium pressure mercury vaporlamp emitting in the ultraviolet and mostly in the wavelength range 200nanometers (nm) to 400 nm, where such a lamp is sold for example byFusion delivering power of 200 watts per centimeter (W/cm), with thecable traveling at a speed of 80 meters per minute (m/min), and withexposure taking place in a single pass, even when the coating has athickness of about 100 μm.

[0046] In a variant, one or other of compositions No. 1 and No. 2 mayalso include pigments, fillers, spreading additives or slidingadditives, adhesion promoters, ultraviolet stabilizers, andantioxidants.

[0047] Table 2 below gives the results of tests carried out inapplication of the ISO 4589-2 protocol for determining the oxygen limitindex (OLI), the self-extinction time, and the combustion length of fivesamples, thereby characterizing their flame-retardant properties.

[0048] Comparative sample No. 1 comprises a layer of power cablesheathing material made up of an ethylene vinyl acetate (EVA) copolymerincluding flame-retardant mineral fillers such as aluminum trihydrateAl₂O₃, 3H₂O.

[0049] Each of samples No. 2 and No. 3 comprises a filled layer of EVAsimilar to that of sample No. 1, but covered in an outer flame-retardantcovering of a material based on a polymer obtained from composition No.1 of the invention.

[0050] Samples Nos. 4 and 5 each comprise a filled EVA layer similar tothat of sample No. 1 and covered in an outer flame-retardant coatingmade of a material based on a polymer obtained from composition No. 2 ofthe invention.

[0051] Because of their outer coatings of the invention, samples Nos. 2,3, and 5 present an OLI that is 5% greater than that of sample No. 1(only 1% greater for sample No. 4).

[0052] The outer coating of the invention on sample No. 4 presents anOLI that is 1% greater than that of sample No. 1. TABLE 2 Thickness ofouter Extinction Combustion Sample No. coating (μm) OLI (%) time (s)length (mm) 1 — 32 >120 10 2  40 37 175 35 3  70 37 170 35 4  50 33 58 55 100 37 103 10

[0053] The use of isobornyl acrylate requires the thickness of thecoating to be increased in order to obtain an OLI of 37%. Nevertheless,from tests performed on sample No. 5, it is observed that this compoundmakes it possible not only to improve the mechanical properties andabrasion resistance of the coating, but also to reduce its combustionlength and to shorten its extinction time.

[0054] Table 3 below gives the results of tests performed on threevertical cables, the tests being carried out using the IEC 3321protocol. The test consists in subjecting a cable or an insulatorconductor in the vertical position to a flame fed with a mixture of airand propane at respective flow rates of 4 liters per minute (/min) and640 milliliters per minute (m/min) coming from a burner that ispositioned at an angle of 45° relative to the vertical axis of thesample and applied for a duration of 60 seconds, and then determiningextinction time and combustion length.

[0055] Cable No. 1 comprises a copper conductor having a section of 16square millimeters (mm²) coated in a flammable insulating layer made ofa compound containing polyethylene cross-linked by the silane processand not containing any flame-retardant mineral fillers.

[0056] Cables No. 2 and No. 3 both comprise a coated copper conductorsimilar to the conductor of cable No. 1, each coated in an insulatinglayer similar to the layer of cable No. 1, said layer in turn beingcoated in an outer layer of flame-retardant coating made of a materialbased on a polymer obtained from composition No. 1 in one case andcomposition No. 2 in the other.

[0057] Polymerization was obtained by means of a 200 W/cm ultravioletlamp and the cable travel speed was 50 m/min. TABLE 3 Thickness of outercoating Extinction Combustion Cable No. (μm) time length (mm) 1 — >4′440 2  41   2′28″ 135 3 100   2′26″  95

[0058] A cable passes the test if the combustion distance above the zonein which the flame was applied does not exceed 425 mm. As expected,cable No. 1 does not pass the test. Furthermore, the combustion lengthof cable No. 3 is shorter than that of cable No. 2.

[0059]FIG. 1 is a cross-section view of a power cable of the invention.

[0060] The cable 1 comprises, by way of example, a transmission element2 such as an electrical conductor, e.g. made of copper, coated in aflammable insulating layer 3 itself coated in a layer 4 offlame-retardant coating made of a material based on a polymer obtained,for example, from composition No. 2 and of thickness that is preferablyequal to about 100 μm.

[0061] The layer 4 of the invention is an outer coating layer on theinsulating layer, since it provides flame-retardant properties andpreferably also resistance to abrasion. Nevertheless, the cable 1 couldnaturally include one or more other layers between the insulating layerand the layer 4 of the invention.

[0062] The flame-retardant coating layer is made by applying thepolymerizable liquid composition on the flammable insulating layer usinga conventional coating technique, e.g. application by means of a brushor by spraying.

[0063] In a variant, the flame-retardant coating layer is made from atape impregnated in the composition and wound on the flammableinsulating layer.

[0064] The invention is equally applicable to any flammable element usedin the manufacture of a telecommunications or a power cable, for examplea reinforcing element, an optical fiber protection tube, a paddingelement, a grooved core, or a braid.

[0065] More generally, the invention is equally applicable to powercables and to telecommunications cables, to data cables, to electricalcables, and to optical fiber cables.

What is claimed is:
 1. A flame-retardant cable comprising: atransmission element; a flammable element; and a flame-retardant coatinglayer surrounding said flammable element, and made of a material basedon a polymer obtained from a polymerizable liquid composition containingat least a precursor for said polymer including functional groupsselected from acrylates, methacrylates, epoxies, vinyl ethers, allylethers, and oxetanes, wherein said material includes at least onephosphorous group.
 2. A flame-retardant cable according to claim 1,wherein said phosphorous group is chemically bonded to said polymer. 3.A flame-retardant cable according to claim 1, wherein the precursor ofsaid polymer includes at least one phosphorous group.
 4. Aflame-retardant cable according to claim 1, wherein said material ishalogen-free.
 5. A flame-retardant cable according to claim 1, whereinsaid flammable element is selected from at least one of the followingelements: an insulating layer; a sheathing layer; a reinforcing element;an optical fiber protection; a padding element; a groove core; a tape;and a braid.
 6. A flame-retardant cable according to claim 1, wherein,when said flammable element is an insulating layer, said insulatinglayer is made from a material selected from a halogen-free thermoplasticpolymer, and preferably a polyethylene.
 7. A flame-retardant cableaccording to claim 1, wherein the transmission element is selected froma conductor of light and a conductor of electricity.
 8. Aflame-retardant cable according to claim 1, wherein said flame-retardantcoating layer is made by applying said polymerizable liquid compositionon said flammable element using a coating technique selected fromspraying, dipping, impregnation, and application by means of a brush. 9.A flame-retardant cable according to claim 1, wherein saidflame-retardant coating layer is made from a tape impregnated in saidpolymerizable liquid composition and wound on said flammable element.10. A flame-retardant cable according to claim 1, wherein saidpolymerizable liquid composition contains a reactive diluant includingan antiabrasion compound, preferably of bicyclic structure andcontaining at least one functional group that is selectively reactivewith one of the functional groups of said polymer precursor.
 11. Aflame-retardant cable according to claim 10, wherein the number of partsby weight of said antiabrasion compound relative to 100 parts by weightof said liquid composition is less than
 95. 12. A flame-retardant cableaccording to claim 10, wherein, when said antiabrasion compound containsat least one acrylate functional group, the acrylate equivalent weightof said antiabrasion compound is greater than
 80. 13. A flame-retardantcable according to claim 1, wherein the liquid composition ispolymerizable by actinic radiation, and when said actinic radiation isof the UV type, the composition includes a photoinitiator.
 14. Aflame-retardant cable according to claim 13, wherein the number of partsby weight of said photoinitiator relative to 100 parts by weight of saidcomposition lies in the range 0.1 to
 10. 15. A flame-retardant cableaccording to claim 1, wherein the liquid composition is polymerizable byUV radiation and contains: 80 parts by weight of said polymer precursor,said precursor being a halogen-free oligomer; 17 parts by weight of anisobornyl acrylate; and 3 parts by weight of a photoinitiator.
 16. Aflame-retardant cable according to claim 11, wherein the number of partsby weight of said antiabrasion compound relative to 100 parts by weightof said liquid composition is in the range 10 to
 30. 17. Aflame-retardant cable according to claim 12, wherein, when saidantiabrasion compound contains at least one acrylate functional group,the acrylate equivalent weight of said antiabrasion compound is about210.
 18. A flame-retardant cable according to claim 14, wherein thenumber of parts by weight of said photoinitiator relative to 100 partsby weight of said composition is about 3.